305/30R20 vs 255/40R20
255/40R20 is outside the ±3% rule as a replacement for 305/30R20: the diameter differs by +3.04%, so the speedometer, odometer and ABS calibration will be off and clearance needs checking.
Speedometer and odometer
With 255/40R20 on a car calibrated for 305/30R20, the speedometer reads 58.2 mph when you're really doing 60, and the odometer under-counts by 3.0%.
| Speedo shows | Actual speed |
|---|---|
| 30 mph | 30.9 mph |
| 45 mph | 46.4 mph |
| 60 mph | 61.8 mph |
| 70 mph | 72.1 mph |
| 80 mph | 82.4 mph |
Other sizes close to 305/30R20
- 305/30R20 vs 255/40R19-0.64%
- 305/30R20 vs 245/40R20+1.88%
- 305/30R20 vs 245/35R20-1.66%
- 305/30R20 vs 255/35R20-0.65%
- 305/30R20 vs 275/35R20+1.37%
- 305/30R20 vs 225/45R19-0.85%
- 305/30R20 vs 235/45R19+0.45%
- 305/30R20 vs 285/35R20+2.39%
- 305/30R20 vs 285/30R20-1.74%
- 305/30R20 vs 275/30R20-2.60%
- 305/30R20 vs 265/35R20+0.36%
- 305/30R20 vs 235/35R20-2.68%
Questions
Can I replace 305/30R20 with 255/40R20?
255/40R20 is outside the ±3% rule as a replacement for 305/30R20: the diameter differs by +3.04%, so the speedometer, odometer and ABS calibration will be off and clearance needs checking.
What's the difference between 305/30R20 and 255/40R20?
255/40R20 is 0.83" taller and 50 mm narrower; the sidewall is +0.41". At an indicated 60 mph you'd actually be doing 61.8 mph.
Will 255/40R20 fit on 305/30R20 wheels?
Both use a 20" rim, so the wheel diameter matches; check the wheel's width range for a 255 mm tire and the fender clearance.